This invention relates generally to optical devices and specifically to a wavelength preserving regenerator for DWDM transmission systems.
Optical communication systems are prevalent in many networks. For example, optical fibers can be used for voice networks, data networks as well as hybrid networks (e.g., networks that carry voice, data, video and possibly other types of information). These systems are especially useful to transport information over long distances.
There is, however, a limit as to the distance a light signal may travel. To increase this distance, optical amplifiers may be used. Optical amplifiers are advantageous in that they boost the optical amplitude of a signal. Unfortunately, optical amplifiers also create and amplify optical noise (termed Amplified Spontaneous Emission noise) and do not overcome transmission impairments such as chromatic and polarization mode dispersion. All of these impairments eventually leads to errors in the detection of signals after propagation and amplification over long distances.
As a result, digital systems also include regenerators. A regenerator reconstructs the original digital pulse signals in shape and timing rather than simply amplifying that signal without regard for reshaping or retiming. A regenerator is thus typically termed a 3R device (Regeneration, Reshaping and Retiming) whereas an optical amplifier is termed a 1R device. For example, in an optical communications system, the regenerator would detect an optical signal and convert that optical signal to an electrical signal. The state (e.g., logical xe2x80x9c0xe2x80x9d or logical xe2x80x9c1xe2x80x9d for a binary signal) is then determined and a new optical signal is generated. Since the new signal has been regenerated, it will be identical to the original signal, assuming that an error has not been introduced (i.e., a logical xe2x80x9c0xe2x80x9d was interpreted as a logical xe2x80x9c1xe2x80x9d, or vice versa).
In one aspect, the present invention provides an improved optical regenerator and systems that can use such a regenerator. One such optical system includes a number of substantially identical, tunable, wavelength-preserving, optical regenerators. Each of the regenerators is tuned to a different wavelength so that when a regenerator is tuned to a given wavelength that regenerator will generate a regenerated, reshaped and retimed optical signal at the same given wavelength. The system also includes a multiplexer coupled to receive and combine regenerated optical signal from each of regenerators.
In the preferred embodiment, the wavelength preserving optical regenerator includes an optical-to-electrical (O/E) conversion unit. This unit will receive an optical signal at a first wavelength. The optical signal has been demultiplexed from a WDM optical signal, possibly by a tunable optical filter. The filtering function may be integrated with the regenerator or external to it. A retiming and regeneration circuit, e.g., a D-type flip/flop and a clock circuit, receives a signal from the O/E unit and provides a regenerated electrical signal to an electrical-to-optical conversion (E/O) unit. The E/O unit generates an optical signal based on the electrical signal. In this embodiment in particular, the E/O unit includes a tunable laser that is tuned so that the wavelength of the optical signal generated by the electrical-to-optical conversion unit has a fixed relationship with (e.g., is equal to) the first wavelength.
In one embodiment, the regenerator includes an optical monitor. The optical monitor takes optical signals from the input of the O/E unit and the output of the E/O unit and compares the wavelengths. Based on this comparison, the optical monitor generates a control signal that is used to tune the wavelength of the optical signal generated by the E/O unit. For example, the output wavelength can be locked onto the input wavelength. This embodiment is useful because the E/O unit is self-tuning, which simplifies installation and maintenance.
The present invention has a number of advantages. For example, the optical regenerators are simple and provide convenient use for WDM systems. The invention provides a useful way to separate out wavelengths, identify and regenerate a pulse stream, and recombine the different channels. In one embodiment, an optical regeneration system can utilize a number of identical regenerators for different wavelength channels. Using a single design of regenerators reduces costs because manufacturing must be adjusted for different designs. Maintenance costs are also reduced because fewer replacement devices need to be kept on hand.